Polishing pads, such as chemical mechanical polishing (CMP) pads are widely used in a semiconductor manufacturing field to horizontally planarize various types of layers, such as oxide layers, nitride layers, metal layers, etc. In one conventional arrangement, a CMP pad is provided with a hole H. A chuck including a wafer to be planarized is placed in contact with the CMP pad including the hole H. A slurry is provided on the polishing pad to facilitate the CMP process and a light reflectance measurement unit is used to determine when the wafer has been sufficiently planarized. The end point of the polishing process is determined by the light reflectance measurement unit by measuring the light reflected through the hole or window H. However, the ability of the slurry to fall through the hole in the CMP pad reduces the accuracy of the measurements made by the light reflectance measurement unit.
In another conventional device, the CMP pad does not include a hole. In such an arrangement, the progress of the polishing cannot be monitored in-situ and a manufacturing delay is introduced when the wafer must be removed from the CMP process to check the progress of the polish. In such a system, the end point of the polishing process may be determined utilizing a preset timing period. However, such systems are inherently inaccurate.
In yet another conventional device, a pad window is inserted in the hole of a top polishing pad. The pad window is made of a transparent material, which allows transmission of the laser beam. However, in the conventional device, the pad window sags in downwardly and/or an interface gap occurs between the top polishing pad and the window due to mechanical polishing pressure. As a result, slurry may accumulate on the top surface of the sagging pad window or slurry may leak through gaps in the side. Each of these causes scattering of the laser beam and degrades the transmission.